Barring major issues, I hope to release the final version of 3.1.0 in
two weeks or so.

For a full list of changes see the changelog. Some of the highlights are:

Edit box improvements

One of the common complaints about the removal of committing changes in
the 3.0 release is that when editing subtitles or translating subtitles
into another language, it's often useful to be able to see the original
version of the line as you work. 3.1 addresses this by adding an option
to show the text of the line when you first selected it in addition to
the current text of the line, as shown above.

There is now a basic character counter for subtitles. This displays the
number of characters on the longest line of the currently selected
subtitle (treating only \N as line breaks), and changes the background
color if the customizable maximum is exceeded.

The styling buttons finally have hotkeys, and split line at cursor can
have a hotkey assigned.

Internationalization

Case-insensitive searching now works correctly with non-latin alphabets,
such as Greek and Cyrillic.

The spellchecker now does a much better job of splitting words at the
right places rather than assuming that all languages use English's
word-splitting rules.

The main edit box now supports dead keys on OS X (but sadly still does
not support IMEs).

Autosave

Unfortunately, sometimes Aegisub crashes when it's been a while since
you last saved. Aegisub has always had a built-in autosave, but the
place it autosaves files to is not very visible, and reopening the
original file could result in the autosave with all of your work getting
overwritten. To solve this, Aegisub now has a convenient way to open
autosaved files from the UI (File -> Open Autosaved File...), and it
keeps older copies of your files around for a while.

Automation

Scripts must once again be UTF-8. Trying to support scripts in arbitrary
encodings caused some problems and was a bad idea in general.

MoonScript is now natively supported, and some
of Aegisub's included libraries are now written in it.

Error reporting when running Automation scripts has been improved. The
log window is much larger so that it's actually somewhat readable, and
a stack trace is printed when errors occur.

Button IDs can be set for Automation dialogs, making it possible for
buttons to be triggered on Esc/Enter

OCD Typesetter Things

Duplicate and Shift by 1 Frame has been replaced with splitting lines at
the current video frame. This allows you to time a line to the full
duration of a sign, then step through the video and on each frame which
the sign changes, hit Ctrl-D to make the current line end on the
previous frame and create a copy of it that starts on the current frame.
For signs which do not change on every frame, this cuts slightly cuts
down on the number of keystrokes needed since you do not need to set the
end times explicitly. If you prefer the old workflow, trying to split a
line when the line isn't actually visible on the current frame will
simply make a duplicate of the line visible only on the current frame,
which is a slightly more general version of the old behavior.

Holding alt now preserves the aspect ratio when adjusting the scale with
the visual tool, and the vector clip tool now supports box selection of
control points. The X/Y rotation's grid is now less uselessly small.

The video colormatrix tag is now actually read from files, so passing
files between people with different settings for Force BT.601 no longer
breaks horribly. Note that Force BT.601 will switch to being off by
default in a future version (once most people have started using
versions of Aegisub with this fix).

About Aegisub

Aegisub is an advanced subtitle editor for Windows, and UNIX-like systems, such as Linux, Mac OS X and BSD. It is open source software and free for any use.

Aegisub natively works with the Advanced SubStation Alpha format (aptly abbreviated ASS) which allows for many advanced effects in the subtitles, apart from just basic timed text. Aegisub's goal is to support using these advanced functions with ease.